1. Field of Invention
The invention relates to a high voltage charge circuit, and particularly to a high voltage charge circuit with the ability of rapid charging.
2. Related Art
When taking a picture with a camera, a user usually uses a flash to provide sufficient light to the external environment. However, the working voltage for a flash, e.g. 300V, is much higher than the DC voltage provided by the camera, e.g. 5V. To solve this problem, a camera is set up with a high voltage charge circuit. The DC voltage (low voltage) is raised to a high voltage using a transformer having a high winding ratio charging a high voltage capacitor. When the high voltage capacitor is charged to match the working voltage of the flash, the high voltage capacitor is used as a source to provide the desired working voltage.
The prior high voltage charge circuit 10 is also referred to as a ring chock converter (RCC). As shown in FIG. 1, the RCC 10 comprises a DC source 12, e.g. 5V, resistors 14, 16 and 30, a power transistor 18, a transformer 20, a diode 22, a high voltage capacitor 24, e.g., 300V, a Zener diode 26 with a break down voltage of 300V, a capacitor 28 and a standby control circuit 32. The transformer 20 has primary side windings N1, secondary side windings N2 and auxiliary side windings N3, where the primary side windings and the auxiliary side windings N3 may induct with the secondary side windings N2. The primary side windings N1 have opposite polarity with the secondary side windings N2 and the secondary side windings N2 have a winding number N times the winding number of the primary side windings N1, e.g., 60 times.
When the DC source 12 provides a current to the transformer 20, the resistor 14 and the primary side windings N1 are turned on and the power transistor 18 is operated in a saturation region. Next, the auxiliary windings N3 and resistor 14 are turned on. At this time, the current flowing through the primary windings N1 is a magnetic current whose energy is stored in the transformer and does not charge the high voltage capacitor 24.
When the current flowing through the resistor 14 gradually increases, the power transistor 18 is operated from the saturation region to the active region to decrease the current flowing through the primary side windings and invert the polarities of the primary and secondary windings N1 and N3. At that time, the power transistor 18 is cut off and the secondary windings N1 and the diode 22 turn on. After the secondary side windings N2 transfer the energy stored in the transformer 20 to the high voltage capacitor 24, the primary side windings N1 go back to their initial state. Then the loop of the resistor 14 becomes conductive again and the entire process repeats.
When the high voltage 24 reaches a predetermined voltage level, e.g. 300V, it enables the Zener diode 26 to break down and leads to a short circuit. At that time, the standby control circuit 32 is triggered to stop the operation of the transformer 20, and the high voltage capacitor 24 is no longer charged.
FIG. 2 is a diagram depicting the relation between the charging current and time. It can be seen from the drawing that when the charging current becomes zero the transformer 20 begins to work, providing a charging current to charge the high voltage capacitor 24.
From the above, it may be known that the prior high voltage charging circuit 10 has the following disadvantages:
1. In the prior art the power transistor 18 is a bi-polar junction transistor (BJT) that when turned on requires an additional driven base current. Since the power transistor has a saturated voltage VCE of about 300 m and consumes a great deal of power, the charging effect may not be satisfactory.
2. Since the transformer 20 requires the winding N3 and the power transistor 18 generally has a switch frequency of 10 kHz, the transformer is not easily miniaturized.
3. Since the prior high voltage charging circuit is not operated in a continuous conduction mode, it may not be efficient enough.